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Discussion Starter #1
Alternator good, Battiers drain

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03 Duramax with Allison. 2500 HD crewcab, 175,000 miles.
always serviced, tires rotated/aligned. Only tow a 21 foot bass boat locally 5 or 6 times a year.
2 weeks ago i tried to unlock doors with key pad but locks wouldnt disengage.
Opened door with key & all electrical was dead.
Connected battery charger & it pegged the charge side.
Batteries were almost 6 years old.
Next day they were charged enough to drive to get new batteries.
drove local about 300 mies in the last 16 days.
This morning same secenerio , all electrical dead, battery charger pegs on charge.
Took altenator to 2 different parts houses today & it tests good.
Any Ideas whats going on ??
 

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having the same problem when its like 25 deg below zero
 

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One battery is bad, so it's draining the good battery. Common in parallel battery setup. Replace both batteries.
 

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There could be a bad diode in your alternator that is draining your battery and cannot be tested for while it is in the truck, I have had this happen to me before. I suggest taking it out and having it tested again.
 

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Discussion Starter #7
havent done anything to truck except required maintenence, tires etc.
never touched anything electrical except batteries.
Parked it one night, next morning batteroes dead. They were about 6 years old so i changed them, 2 weeks later, key pad wouldnt open the doors, so used key, everything dead, no radio, no dome light, nothing.
took alternator to 2 parts houses & it tests ok.
Charged batteries again, 2 days later, everything dead again.
 

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I would also take it out and have it tested.
 

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two things that I see could be an issue is bad diode bridge in alt and sometimes those machines dont pick it up.... test could be to after you shut the truck off remove the charge wire off the alt and see if battery drains.... if it does replace alt.

two is check your ground on the passenger side of block remove inner fender liner and you'll see it clear as day... I have seen those wires break. make sure you check the connections to the battery also side posts are notorious for battery acid going down and corroding the wires
 

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Bench Test Alternator. Charge batteries with separate charger. Load test batteries. Once the alternator is confirmed good and the batteries load test good, what is the charging voltage across the batteries after engine is warmed? Should be 13.8 VDC to 14.2 VDC. If the voltage is not good, then you have a ground problem or a short to ground. If the alternator tests good out of the vehicle, and you have good charging voltage across the batteries, then you are looking for a load on them. This gets a bit more intricate. But essentially, I would put an ammeter in the positive circuit off a battery to the vehicle and start pulling relays or fuses looking for the current to drop as each relay or fuse is pulled. You have to pull the Mega-fuse on the Intake Air heater and Glow plugs. There may be a few other low current circuits that are only fused also. Or, ... you could use a test lamp or DVM downstream of the relays looking for voltage. But, my bet is on the alternator. You really have to take them out of the vehicle and have them bench tested to find any short in them.
 

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thats strange. same thing happened with my truck. the odd time i would go out to start it. and it wouldnt have enough power to start. they were the original batteries. so i put 2 new ac delco 800cca batteries in it. but the meter still goes up and down so i belive its my altenator that was screwed in the first place
 

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LT is on the right track if it's the alternator taking it to parts house to check probably won't do you any good. They can check to see if it's charging but don't know how to check for a drain. If the alternator is draining the system most likely its a positive diode shorted turning on the regulator it can still charge at 14v. You can check it out quicker on the truck. Disconnect the negative cables on both batteries. In between one of the batteries connect a test light from the (-) cable to the (-) post if the light is on there's a drain on the system. Make sure your doors are closed if there's a hood light pull it out. Remove the 10ga. wire from the back of the alternator if the light goes out you found the drain, if not start pulling fuse's and relays. If you make a test light with long leads on it you can take it into the cab while pulling fuse's.
 

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Not sure if this thread is dead but I'd like to add my two cents. Hopefully it helps someone along the line.

I have a 2001 2500HD and my batteries were draining even though the alternator was running fine around 16 amps. Knowing there was a drain I hooked a Fluke multimeter, to view amperage, to the main battery. To do this, disconnect the negative post and attach one of the multimeter's clips to the neg wire. Attach the other clip to the positive post (which remains screwed in). The truck's computer stays on for a while after you close the doors so wait at least 10 minutes after last closing the door to check the multimeter.

It is alright if the truck is drawing around 0.01 amps but in my case it was pulling around 0.70-2 amps. Start pulling fuses under the hood while monitoring the drain. Some of the fuses will show a drop in amperage for few seconds after pulling and then jump back up so pay attention. On my truck, it turned out to be the radio that was drawing power. The remedy was a simple rewiring of the radio's power to the ignition.
 

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in my case it was starter silinoid . replaced starterno more battery trouble
 

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This is a great thread.

My 3 year warranty on my Optima's expires this week.

So, I'll probably be reading these posts again.:thumb

Thanks!
 

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Was a Heavy Duty Mech for 20 years and taught apprentices for 10. A quick and dirty way to test for parasitic load is to hook up an ammeter to your system (use an inline fuse as part of your test rig) placed in the ground side of your system (this way you cannot accidentally short out if your rig touches the body. If you see an amperage draw with the vehicle shut off you know you have a parasitic load.

Now one by one pull your fuses.Check the draw with each fuse you pull. Mark down the draw so you have record of your test. Reinstall each fuse before pulling the next. Repeat until you find the circuit that causes a major drop in current (remember most modern vehicles have a small draw at all times due to the security / remote start features). If you have a blown alternator diode you will not be able to remove the draw unless you disconnect the fusible link.

Final test, remove battery ground cable, remove ALL wiring from alternator (tape up each connection so you do not cause a short. Reconnect ground cable has parasitic load dropped substantially? If so you know for certain that you have a bad diode (bad bridge or isolation diode) in alternator. If you have dual battery set up disconnect one battery and do tests. If you do not show parasitic draw, reconnect other battery and see if parasitic draw shows up. If so probable cause is battery isolation diode bridge shot.

This takes about 15 minutes to do, you do NOT start the vehicle and you do not have to remove any major components to determine where the problem is. All you do is isolate circuits until you have found the culprit.
 

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I know this is an old thread but i have a 2007 lmm and my batteries die over night they are new iv done the tests in the comments above with no luck i unhook the - on both batteries and put a test light between cable and battery and its lit up unhook alt and its still lit up but does dim a hair then i pulled every fuse in the truck and non of them make the light go out. anyone have an idea on whats causing this havent added any new wiring in years and everything that is in there like lights radio and lift pump are on keyed on fuses.
 

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Get an amp clamp and put it on the positive cable. see what the draw is with everything off. If I remember corretly 1/2 amp is equivalant too one light bulb. So if you have an amp draw pull fuses and see if the amp clamp changes
 

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I know this is an old thread but i have a 2007 lmm and my batteries die over night they are new iv done the tests in the comments above with no luck i unhook the - on both batteries and put a test light between cable and battery and its lit up unhook alt and its still lit up but does dim a hair then i pulled every fuse in the truck and non of them make the light go out. anyone have an idea on whats causing this havent added any new wiring in years and everything that is in there like lights radio and lift pump are on keyed on fuses.
Have you had the recalls completed?

2007 GMC Sierra 2500HD Recalls - Cars.com

or

http://www-odi.nhtsa.dot.gov/owners/SearchResults;jsessionid=9VC4VTtL6p1MnpT2yR1G2wT0h9pcLpnyGGYM5YvvDVQmGRh5d2VK!-942636767?prodType=V&searchType=PROD&targetCategory=A&searchCriteria.model=SIERRA+3500&stats=451859%2C1%2C0%2C4%2C15%2CSIERRA+3500&makeStats=450765%2C6%2C2%2C189%2C67%2CACADIA%3B859805%2C0%2C0%2C0%2C8%2CC+SERIES%3B1042773%2C0%2C0%2C0%2C3%2CC+SERIES+C4+4500%3B1042772%2C0%2C0%2C0%2C3%2CC+SERIES+C5+5500%3B1973149%2C1%2C0%2C0%2C0%2CC5500%3B678928%2C0%2C0%2C1%2C6%2CC6500%3B678930%2C0%2C0%2C0%2C6%2CC7500%3B678931%2C0%2C0%2C0%2C4%2CC8500%3B451852%2C3%2C0%2C14%2C37%2CCANYON%3B506777%2C0%2C0%2C7%2C26%2CENVOY%3B491781%2C0%2C0%2C0%2C7%2CENVOY+360%3B685769%2C0%2C0%2C0%2C5%2CEXPRESS%3B505774%2C6%2C0%2C1%2C41%2CSAVANA%3B451853%2C0%2C0%2C0%2C5%2CSAVANA+1500%3B451854%2C0%2C0%2C1%2C5%2CSAVANA+2500%3B451856%2C0%2C0%2C1%2C7%2CSAVANA+3500%3B370641%2C1%2C0%2C25%2C126%2CSIERRA%3B451857%2C0%2C0%2C30%2C9%2CSIERRA+1500%3B451858%2C1%2C0%2C15%2C15%2CSIERRA+2500%3B451859%2C1%2C0%2C4%2C15%2CSIERRA+3500%3B506786%2C0%2C0%2C1%2C16%2CSIERRA+DENALI%3B738771%2C0%2C0%2C0%2C16%2CSIERRA+HYBRID%3B859799%2C0%2C0%2C1%2C7%2CT+SERIES%3B923783%2C0%2C0%2C0%2C2%2CT+SERIES+C6+6500%3B923778%2C0%2C0%2C0%2C2%2CT+SERIES+C7+7500%3B923775%2C0%2C0%2C0%2C2%2CT+SERIES+C8+8500%3B708780%2C0%2C0%2C0%2C5%2CT6500%3B708778%2C0%2C0%2C0%2C5%2CT7500%3B708776%2C0%2C0%2C0%2C4%2CT8500%3B724771%2C0%2C0%2C0%2C10%2CTERRAIN%3B505772%2C0%2C0%2C0%2C31%2CTOPKICK%3B678909%2C1%2C0%2C0%2C20%2CW-SERIES%3B567795%2C1%2C0%2C0%2C10%2CW3500%3B567796%2C2%2C0%2C0%2C10%2CW4500%3B567797%2C1%2C0%2C0%2C9%2CW5500%3B450767%2C2%2C0%2C18%2C105%2CYUKON%3B370644%2C0%2C0%2C15%2C57%2CYUKON+DENALI%3B370646%2C0%2C0%2C5%2C55%2CYUKON+DENALI+XL%3B657816%2C0%2C0%2C6%2C20%2CYUKON+HYBRID%3B370645%2C2%2C0%2C7%2C63%2CYUKON+XL%3B451860%2C0%2C0%2C3%2C24%2CYUKON+XL+1500%3B451861%2C0%2C0%2C0%2C15%2CYUKON+XL+2500%3B&jsonBaseURL=%2Fdownloads%2Ffolders%2F&searchCriteria.model_yr=2009&searchCriteria.make=GMC&searchCriteria.prod_ids=451859
 

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